Method for manufacturing a fuel nozzle blank with a metallic cladding

ABSTRACT

A method for manufacturing a metallic body having a core and metallic cladding by Hot Isostatic Pressing includes the step of providing a hollow body that has a bottom wall, a core and a lateral wall, wherein the hollow body is filled with cladding material. The bottom wall and the top wall of the hollow body, prior to the step of HIP, are provided with at least one centering means for centering a final body obtained in the step HIP in a metal machining apparatus.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a metallic body having a cladding according to the preamble of claim 1.

BACKGROUND ART

Hot Isostatic Pressing (HIP) is a conventional method for manufacturing components of metallic material. The method allows for manufacturing of complex components in near-net shape and also for integration of different materials in the same product. In HIP, a steel capsule which defines the final shape of the component is filled with metallic powder and thereafter subjected to high temperature and high pressure so that the particles of the metallic powder bond into a solid component.

Hot Isostatic Pressing may be used to apply claddings of metallic materials onto pre-manufactured cores. WO2004/030850A1 describes a method for manufacturing fuel valve nozzles. According to the method, a metallic tube section is arranged to form a space around a pre-forged nozzle core. The space is filled with metallic powder and the arrangement is enclosed in a capsule and subjected to HIP so that the metallic powder, the core and the tube section bond into a solid component.

A similar method for manufacturing a valve nozzle is described in Applicants European Patent Application EP12173411. This method comprises the steps of forming a solid blank in a metal machining operation into a hollow body which comprises a bottom wall from which a core extends and a lateral wall which encloses a space around core. The space is filled with metal cladding material and closed by an upper wall and subsequently subjected to HIP.

After HIP the solid components are typically subjected to machining in order to expose the cladding on the core. Typically, machining is performed by turning or milling.

However, often the final consolidated component is deformed during the HIP process. This causes a problem in the machining of the component since it becomes difficult to accurately clamp and center the component in the machining apparatus. As a consequence thereof, the cladding may not be machined to an accurate thickness. A further drawback with the prior art is that the machining of the components is time consuming and costly due to cumbersome manual labor and a poor yield of acceptable components.

Consequently, it is an object of the present invention to present an improved method which allows for manufacturing by HIP of metallic components having a cladding whereby the cladding on the final components has very low thickness variation. A further object of the present invention is to achieve a cost effective method for manufacturing of metallic components having a cladding. Yet a further object of the present invention is to present a method for manufacturing of metallic component having a cladding whereby the method can be performed in short time and with little effort.

SUMMARY OF THE INVENTION

According to a first aspect of the invention at least one of the above objects is achieved by a method for manufacturing a metallic body 50 having a core 5 and metallic cladding 60, comprising the following steps:

-   -   providing at least one a hollow body 2 that comprises a bottom         wall 3, a core 5 that extends from the bottom wall 3 and a         lateral wall 4 that extends from said bottom wall 3 and encloses         the core 5 so that an inner space 6 is formed around the core 5;     -   filling the inner space 6 with a metallic cladding material 8;     -   closing the inner space 6 by arranging a top wall 9 on the         lateral wall 4;     -   positioning the filled hollow body 2 in a capsule 10 and         evacuating air from the capsule (10) and sealing the capsule 10;     -   subjecting the capsule 10 to Hot Isostatic Pressing (HIP) at a         predetermined temperature, a predetermined pressure and for a         predetermined time so that the cladding material 8 and the         hollow body 2 bond and forms a solid body 20;     -   machining the solid body 20 in a metal machining apparatus 30,         wherein at least a portion of the lateral wall 4 is removed and         the exposed cladding material 8 is machined to a cladding 60 of         a predetermined thickness; characterized in that the bottom wall         3 and the top wall 9 of the hollow body 2, prior to the step of         Hot Isostatic Pressing, each are provided with at least one         centering means 11, 12 for centering the final body 20 obtained         in the step of Hot Isostatic Pressing in the metal machining         apparatus 30.

By providing the centering means in the hollow body prior to the step of Hot Isostatic Pressing it is possible to accurately center the HIP:ed solid body in a metal machining apparatus with respect to the center of core of the solid body, even if the solid body is deformed during HIP. By subsequently machining the cladding to a predetermined thickness which is determined as a distance from the center of the core, the thickness of the cladding around the core may be held within a very narrow tolerance range.

The principle of invention is further explained with reference to FIG. 8. FIG. 8 shows schematically a longitudinal cross-section of a solid HIP:ed component 20 comprising a hollow body 2 comprising a bottom wall 3, an upper wall 9 and a core 5. The core 5 is embedded in a cladding material 8. A capsule 10 surrounds the solid body. Centering means 11, 12 in the form of a protruding truncated cone and a truncated cone recess are provided in the bottom and upper walls 3, 9.

FIG. 8, indicates schematically the deformation that has occurred during the HIP process. This deformation is to a certain extent often anisotropic and in particularly in the case of elongated cylindrical components, the periphery of the HIP:ed body may be unevenly deformed.

It should however be appreciated that FIG. 8 is schematic and that the anisotropic nature of the deformation is strongly exaggerated for illustrative reason. In reality the deformation is also much more complicated.

According to the invention, the centering means 11 and 12 are applied prior to HIP in the center of the bottom and upper walls 3, 9 of the hollow body (position X1). During densification, the periphery of the capsule 10 and the solid body 20 is deformed anisotropic in radial direction as indicated in FIG. 8. However, the positions of the centering means 11, 12 are not affected by the deformation. When the solid body subsequently is subjected to a machining operation in the form of turning in a lathe, the solid body 2 may be centered along the line X1 by corresponding centers in the metal machining apparatus. The solid body 2 will then be centered with respect to the true center of the core 5 and the machining operation will yield a cladding with a very small thickness variation around the core.

In the case of conventional manufacturing of a cladded component (which does not comprise centering means), the end of the solid body 20 is typically gripped by a chuck and the solid body will therefore be centered with respect to the center of the chuck. However, since the circumference of the solid body is deformed anisotropic, the center of the chuck will not be aligned with the center of the core of the pre-manufactured body. Instead, the solid body will be centered along the line X2 which is offset from the center of the core. When the solid body is machined the offset centering will cause the solid body to rotate eccentrically and cause the thickness to vary on the core.

Further embodiments and advantages of the present invention are disclosed in the dependent claims and the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1-7 shows the steps of the inventive method.

FIG. 8 shows schematically a hollow body manufactured according to the invention

FIG. 9 shows a fuel valve nozzle blank that has been obtained by the inventive method.

DETAILED DESCRIPTION OF THE INVENTION

The inventive method will in the following be described in detail with reference to the manufacturing of a fuel injection nozzle blank for a diesel engine, in particular a two-stroke diesel engine. For further details it is referred to Applicants non-published European Patent Application EP12173411 which content herewith is incorporated by reference

In a first step of the method a hollow body 2 is provided. FIG. 1 show schematically a side view of the hollow body 2 which is cylindrical and has a rotational symmetric form. The hollow body 2 may be manufactured in tool steel, for example AISI H13/SS2242. The hollow body 2 has a bottom wall 3 from which a core 5 and a lateral wall 4 extends. The core 5 extends from the center of the bottom wall 3 and the lateral wall 4 extends from the periphery of the bottom wall. The lateral wall 4 surrounds the core 5, i.e. is coaxial with the core 5, so that a space 6 is limited between the inner periphery of the wall 4 and the outer periphery of the core 5. In longitudinal direction the lateral wall 4 extends beyond the core 5. The inner periphery of the lateral wall 4 thereby determines the limits of the space 6 in the radial direction and the extension of the lateral wall 4 in longitudinal direction determines the upper extension of the space 6. The bottom wall 3 has an end surface 3 a which is substantial flat so that the hollow body 2 may be placed upright on the bottom of a HIP capsule. An advantage by using a hollow body in the manufacturing of cladded components is that that the provision of a lateral wall 4 results in a more homogenous deformation on radial direction of the hollow body during HIP. This in turn results in low thickness variation of the cladding. A further advantage is that filled hollow bodies may be prepared in advance which is favorable for efficient manufacturing of several nozzle blanks simultaneously.

According to a first alternative of the inventive method, the hollow body 2 is manufactured by machining of a solid blank of metal, i.e. a single piece of metal for example a solid cylindrical bar of steel. The solid steel blank is subjected to a metal machining operation, for example milling, in which metal is removed from the blank such that the core 5, the space 6 and the lateral wall 4 are formed in one end of the blank 1 and leaves the bottom wall 3 in the other end of the blank. The advantage thereof is that the position of the various parts of the hollow body, i.e. the core and the bottom- and upper wall can be made very precise in relation to each other. There is further no need for auxiliary operations such as welding for attaching the parts of the hollow body to each other.

According to a second alternative of the inventive method, see FIG. 3, the hollow body 2 is formed by attaching a tube section 19 onto a preformed nozzle pre-body 1. The nozzle pre-body 1 may for example be manufactured by forging or casting, possibly in combination with machining. The nozzle-pre body comprises a core 5 and bottom wall 3. The upper portion of the bottom wall 3 comprises a shoulder 3 b which surrounds the base of the core 5 and provides thereby a support surface for the tube section 19. The tube section 19 is arranged so that one of its end surfaces is supported on the shoulder 3 a and so that the tube section surrounds the core 5 and extends in longitudinal direction beyond the core. The tube section 19 is thereby a lateral wall 4 and forms a space 6 around the core 5. Preferably, the end of the tube section is welded to the bottom wall 3 in order to keep it in stable position during HIP. This way of manufacturing the pre-manufactured body is fast and inexpensive. The tube section 19 may be manufactured in construction steel, such as 1312 (E235).

In a second step, see FIG. 3, the space 6 is filled with metallic cladding material 8 so that the core 5 is embedded in metallic cladding material. Preferably, the metallic cladding material 8 is a metal powder. The advantage of using powder is that the space 6 thereby easily can be filled even if the core has a complicated form. The metallic cladding material 8 has a different chemical composition than the core 5. The present embodiment relates to a fuel injection nozzle and the purpose of the metallic cladding material is to provide a corrosion resistant layer on the core part of the nozzle. Therefore it is preferred that the metallic cladding material consists of a nickel-based alloy, for example NiCr49Nb1 or NiCr22W6Al5 or NiCr22MoNbTi. After filling, the metallic cladding material 8 may be compacted by stamping or shaking to ensure that all voids are filled in the hollow body (not shown).

In a third step the filled hollow body 2 is closed. Thereby is a top wall 9 arranged on top of the upper end of the lateral wall 4. The top wall 9, see FIG. 3, comprises a lower side 9 b, which is supported on the upper end of the lateral wall 4 and an upper side 9 a which is directed away from the hollow body 2. The upper wall 9 is designed such that it completely covers the upper end of the hollow body 2 and thereby forms a lid which closes the hollow body. In order to allow air to be expelled from the hollow body in a later air evacuation step, the upper wall 9 shall not be sealed to the upper end of the lateral wall. 4. The top wall 9 may be manufactured in construction steel, such as 1312 (E235).

According to the invention, the bottom wall 3 and the top wall 9 of the filled hollow body 2 are provided with centering means 11 and 12 for centering the final HIP:ed body in a metal machining apparatus. FIG. 4 show schematically the position of the centering means in the filled hollow body 2. A first centering means 11 is provided in the end surface 3 b of the bottom wall 3 and second centering means 12 is provided in the upper side 9 a of the top wall 9.

Obviously, the centering means should be provided in a position of the hollow body which is not covered by cladding material, thus free of cladding material.

Preferably, the centering means 11 and 12 are located in the center of the bottom and top wall so that they are aligned along a straight line 13 running through longitudinally through the center of the core 5 and through the both centering means 11, 12.

The centering means in both the bottom wall and the top wall of the hollow body are preferably designed to be engaged by corresponding centers in conventional metal machining apparatuses. According to the present invention, a “metal machining apparatus” also known as “metal machine tool” or “machine tool” may be a metal cutting machine such as a lathe or milling cutter. The metal machining apparatus may also be an Electrical Discharge Machining device.

Preferably, the metal machining apparatus is a lathe, i.e. an apparatus for machining steel by turning. As will be described further below the centers for lathes are so called “male centers” in form of cones or truncated cones. Alternatively, the centers in lathes are so called “female centers” in the form of a sleeve with a conically shaped opening a.k.a “tapered sleeve”. Such centers are commercially available, for example from the company Röhm (RÖHM GmbH, Heinrich-Röhm-Straβe 50, 89567 Sontheim/Brenz, Germany).

Consequently, the centering means in the hollow body are in the form of “male centering means” or “female centering means” The male centering means is a protruding element, for example in the form of a cone or a truncated cone. The female centering means is a recess i.e. a bore. For example the female centering means is in the form of a recess or a bore with the shape of a cone or a truncated cone. It is preferred that the centering means in the hollow body are complementary with commercially available centers in lathes. However, the male centering means could be a protruding element of any shape and the female centering means could be a recess of any shape.

In FIG. 4, a female centering means 11 in the form of a truncated cone shape recess is provided in the end surface 3 a of the bottom wall 3. It is preferred to provide a female centering means in the lower end surface of the hollow body 2 since the hollow body 2 then may be placed steadily in upright position. A male centering means 12, in the form of a protruding truncated cone is provided in the upper surface 9 a of the upper wall 9.

It is obvious that either a male centering means or a female centering means could be provided in the upper wall or in the bottom wall of the hollow body 2. For example, a male centering means could be provided in the bottom wall 3 and a female centering means in the upper wall 9 or vice versa. It is also possible to provide male centering means in both the bottom wall 3 and the upper wall 9. Or to provide female centering means in both the bottom wall 3 and the upper wall 9 of the hollow body 2.

Female centering means, e.g. recesses or bores, may be achieved by drilling or milling. Male centering means, for example cones, or truncated cones, may be achieved by machining of the end surface of the bottom wall or the top wall of the hollow body. It is also possible to pre-manufacture cones or truncated cones, by machining and subsequently attaching the cones by welding to the top or the bottom walls of the hollow body.

The hollow body is then placed in a capsule 10, see FIG. 5. The capsule is preferably a steel tube with a closed bottom end. Preferably, the steel tube is manufactured from low-carbon steel. The length of the capsule is larger than the length of at least one hollow body. However, as is shown in FIG. 5, a plurality of hollow bodies are typically HIP:ed simultaneously in one single capsule. The plurality of hollow bodies could be any number, such as 2 or more, 5 or more or 10 or more. For example the plurality of hollow bodies could be 2-10 or 2-20. The length of the capsule is therefore larger than the total length of the plurality of the hollow bodies that shall be subjected to HIP, i.e. the sum of the length of the individual hollow bodies. The wall thickness of the tubular capsule is large enough (normally at least 1 mm) to ensure that the interior of the capsule is sealed during HIP. The inner diameter of the capsule is slightly larger than the outer diameter of the hollow bodies. For example, the ratio between the inner diameter of the capsule and the outer diameter of the hollow body defined as D_(capsule)/D_(hollow body) is in the range of 1-1.10.

With reference to FIG. 5, the plurality of hollow bodies 2 are stapled on top of each other in the capsule 10. To avoid bonding of adjacent hollow bodies to each other, the top and bottom surfaces 3 a, 9 a of the hollow bodies are provided with a coating which prevents metallurgical bonding. For example the coating is boron nitride.

There is a possibility that female centering means in the hollow bodies deforms or even closes during HIP. To avoid this, a cover piece 40 may be placed in the bottom of the capsule, prior to inserting the hollow bodies into the capsule. The cover piece 40 comprises one surface 40 a which is supported on the bottom of the capsule and one surface 40 b which comprises a protruding element 40 c which is adopted to fit into a female centering means. In this case the protruding element 40 c is truncated cone. The protruding element 40 c of the cover piece 40 fills out the female centering means 11 of the first hollow body 2 in the capsule 10 and prevents the centering means 11 from deforming during HIP. Obviously, the cover piece is also provided with a boron nitride coating to prevent bonding. Further filled and closed hollow bodies are subsequently inserted into the capsule and stapled on top of each other. Thereby is the male centering means 12 of one hollow body 2 received in the female centering means 11 of the next hollow body 2. On top of the uppermost hollow body 2, a second cover piece 40 is placed. The second end piece comprises one surface 40 b which comprises a recess 40 d which is adopted to receive the male centering 12 means of the uppermost hollow body. The opposite surface 40 a is flat and directed towards the opening of the capsule. The arrangement of the second cover piece 40 prevents that the male centering means of the uppermost hollow body from damaging the capsule during HIP. It is obvious that the design of the cover pieces may be adapted to the centering means in the hollow bodies.

The described arrangement of placing several hollow bodies in a capsule is of course a cost effective way of manufacturing large amounts of injection nozzles. However, the described arrangement of hollow bodies with female centering means in bottom and male centering means in the top provides additional advantages. Firstly, this arrangement locks individual hollow bodies in the staple to each other and causes the staple of hollow bodies to remain relatively stable during HIP. Secondly, by the described arrangement the female centering means in one hollow body is protected from deformation during HIP by the male centering means in another hollow body. Therefore only one cover piece is needed to protect the female centering means in the lower most hollow body. This reduces cost further.

Preferably, the male centering means have the form of a truncated cone with an inclination angle of maximum 60°, preferably 40-60°. Preferably, the female centering means is a bore with the same shape of truncated cone, i.e. an inclination angle of maximum 60°, preferably 40-60°. Tests has shown that mating centering means with theses dimensions results in little or no deformation of the female centering means during HIP. Tests have also shown these dimensions prevents that the male centering means to get stuck in the female centering means during HIP. Thereby the hollow bodies may be easily separated from each other after HIP.

When all the hollow bodies have been positioned in the capsule, a lid 10 b with an opening 10 c is welded over the upper end of the capsule. The capsule may comprise air which is has a negative impact on the bonding of the cladding to the core. Therefore the air is evacuated from the capsule 10 by drawing a vacuum in the capsule. The vacuum is drawn through the opening in the lid and subsequently the opening in the lid is welded shut so that the capsule is sealed.

Thereafter, the hollow body 2 is subjected to Hot Isostatic Pressing (HIP), see FIG. 6. The capsule with the hollow body is thereby placed in a HIP furnace 100 and subjected to a predetermined temperature, a predetermined pressure for a predetermined period of time so that the metallic cladding material, the core, the lateral wall and the top and bottom walls bond to each other into a dense and solid final body. Typically, the pressure in the furnace is in the range of 700-1100 bar, preferably, 900-1100 bar, and most preferably around 1000 bar. The temperature is selected to below the melting point of the material with the lowest melting point. The closer the temperature is to the melting point, the higher is the risk for the formation of melted phases in which brittle streaks could be formed. However at low temperatures, the diffusion process slows down and the HIP:ed material will contain residual porosity and the metallic bond between materials become weak. Consequently, the temperature is in the range of 900-1200° C., preferably 1100-1200° C., and most preferably around 1150° C. The duration of the HIP process depends on the size of the components, however short times are preferred for efficient productivity. Therefore the duration of the HIP-step, once said pressure and temperature has been reached, is in the range of 1-4 hours. After the HIP process has been completed, said solid body may preferably be subjected to any suitable heat treatment, such as annealing. After HIP, the solid bodies are separated by cutting the capsule. The capsule may be removed from the individual solid bodies, for example by pickling. The capsule may also remain on the individual solid bodies and instead be removed during machining. FIG. 8 shows the solid body after HIP.

In the final step of the method, the solid body resulting from the HIP process is subjected to a metal machining operation in which at least a portion of lateral wall 4 is removed and the exposed cladding material 8 is machined to a cladding 60 of a predetermined thickness. Typically the machining operation is performed by turning in a lathe.

FIG. 7 shows schematically a metal machining apparatus 30 in the form of a lathe 30. The lathe comprises a head stock 31 to which a face driver 32 is connected. The face driver 32 is rotated by the drive unit of the lathe (not shown) and engages the solid body 20 to rotate it during milling. To engage the solid body, the face driver 32 is provided with hardened drive pins 33 which bite into the end surface 3 a of the solid body 20 so that the rotational movement of the face driver is transferred to the solid body 20. In the center of the face driver a male center 34 in the form of a truncated cone is located. The male center of the lathe is adopted to engage the female centering means 11 of the solid body. Hence, a center in the metal machining apparatus is designed so that it may engage a centering means in the solid body and vice versa.

The tailstock 35 of the lathe comprises a female center 36 which consists of a tapered sleeve 37 with an inner shape in the form of a truncated cone. The sleeve is adopted to receive the male centering means 12 in the top wall 9 of the solid body 20. The center further comprises a shaft (not shown) by which it is attached to the tail stock of the lathe. In this case the center is a live center which is rotatable arranged in the tailstock. However, it could also be a so called dead center. A metal cutting tool 38, i.e. a lathe tool or lathe steel is provided to remove metal from the solid body.

In operation the male center 34 of the face drive is inserted into the female centering means 11 in the first end surface 3 a of the solid body and the female center 37 of the tailstock of the lathe receives the male centering means 12 in the second end surface 4 a of the solid body 20. The face driver presses the solid body towards the female center in the tailstock of the lathe and simultaneously the drive pins 33 are forced into the end surface 3 a of the solid body. The solid body is centered in the lathe when both the male and female centers of the lathe are in engagement with the male and female centering means of the solid body.

If necessary, the centring means 11, 12 in the solid body 20 may be exposed prior to centring the solid body in the lathe. For example, by removing a portion of the capsule by grinding with a hand held tool.

After centering of the solid body, turning is performed until a cladding of desired thickness is achieved. This is achieved in that the control system of the lathe is programmed with a pre-determined distance between the center of the pre-manufactured body and the lathe tool. During turning at least a portion of lateral wall is 4 is removed by the lathe cutting tool 38. Typically the entire lateral wall is removed. The exposed cladding material is then also removed by the lathe tool until the pre-determined distance is reached and a cladding of a predetermined thickness is obtained.

The injection nozzle blank may thereafter be subjected to further machining into a final injection nozzle, for example drilling of holes and further machining of the cladding.

Example

The invention will in the following be described with reference to a comparative example.

Six injection nozzles blanks were manufactured according to the inventive method. The injection nozzles comprised a hollow body of the steel AISI H13/SS2242 filled with a cladding material of NiCr22MoNbTi in powder form. The nozzles blanks had the following dimensions, see FIG. 19: Height (H) 62.7 mm, base diameter (BD): 32.5 mm, upper diameter (UD): 21.5 mm.

The pre-manufactured bodies that were used in the manufacturing of the nozzle blanks were provided with a male centering means in the form of a truncated cone in the top and female centering means also in the form of a truncated cone in the bottom. The male centering means had a base diameter of 11 mm, a height of 5.5 mm and an inclination angle of 60°.

The six pre-manufactured bodies were place in a capsule and HIP:ed for 1 hour at a pressure of 970 bar and a temperature of 1150° C. Subsequently the solid bodies were machined in a lathe of the type Okuma Space Turn LB3000EX. The lathe was provided with centers in face driver and tail stock which corresponded to the centering means in the top and bottom walls of the solid bodies.

After turning the nozzles were cut at a position of 14-16 mm from the top and the cut surfaces were investigated in a light microscope to determine the thickness of the cladding layer. The dashed line in FIG. 9 shows the position for the cut. Four measurements were made around the cut surface and the maximum and minimum readings were recorded.

As comparison, six further nozzles were manufactured and subjected to measurement as described above. However, the pre-manufactured bodies for these nozzles were not provided with centering means. The comparative nozzles were also turned in a lathe as described above but were clamped by a chuck in one end of the solid body.

The results from the measurements are shown in table 1:

TABLE Cladding thickness of test nozzles Comparative Nozzles Inventive Nozzles Sample Max Min Difference Sample Max Min Difference 1 3.26 2.66 0.60 1 3.07 2.95 0.12 2 3.13 2.82 0.31 2 3.00 2.94 0.06 3 3.12 2.80 0.32 3 3.04 2.92 0.12 4 3.13 2.78 0.35 4 2.98 2.93 0.05 5 3.45 2.50 0.95 5 2.98 2.95 0.03 6 3.05 2.89 0.16 6 3.01 2.92 0.09

The results show that, in comparison with the conventionally manufactured nozzles, a much narrower tolerance of the thickness of the cladding is achieved in the nozzles manufactured by the inventive method. The targeted cladding thickness of the inventive nozzles is 3 mm and the measured variation in thickness around the core is in the range of 0.03-0.12 mm. In comparison the comparative nozzles have cladding thickness which varies in the range of 0.16-0.95 mm around the core. 

1. A method for manufacturing a metallic body having a core and metallic cladding, comprising the steps of: providing at least one a hollow body that a bottom wall, a core that extends from the bottom wall and a lateral wall that extends from said bottom wall and encloses the core so that an inner space is formed around the core; filling the inner space with a metallic cladding material; closing the inner space by arranging a top wall on the lateral wall; positioning the filled hollow body in a capsule, evacuating air from the capsule and sealing the capsule; subjecting the capsule to Hot Isostatic Pressing (HIP) at a predetermined temperature, a predetermined pressure and for a predetermined time so that the cladding material bonds to the hollow body and a solid body is formed. machining the solid body in a metal machining apparatus wherein at least a portion of the lateral wall is removed and the exposed cladding material is machined to a cladding of a predetermined thickness; and the bottom wall and the top wall of the hollow body, prior to the step of Hot Isostatic Pressing, each are provided with at least one centering means for centering the final body obtained from Hot Isostatic Pressing in the metal machining apparatus.
 2. The method according to claim 1, wherein the final body is centered in the metal machining apparatus by engagement between the centering means in the final body and corresponding centers in the metal machining apparatus.
 3. The method according to claim 1, wherein the centering means is a male centering means or a female centering means.
 4. The method according to claim 3, wherein the male centering means is a cone or a truncated cone.
 5. The method according to claim 3, wherein the female centering means is a recess having the shape of a cone or a recess having the shape of a truncated cone.
 6. The method according to claim 1, wherein the centering means, prior to Hot Isostatic Pressing, are provided in a center of the bottom wall and a center of the top wall of the hollow body and are aligned along a perpendicular axis which extends through the center of the core of the hollow body and both centering means.
 7. The method according to claim 1, further comprising the step of arranging at least a first cover piece in the capsule, wherein the cover piece includes a protrusion arranged to be fully received in a female centering means or wherein the cover piece includes a recess arranged to fully receive a male centering means.
 8. The method according to claim 1, wherein the hollow body is formed by a metal machining operation in which metal is removed from a solid blank.
 9. The method according to claim 1, wherein the hollow body is manufactured by attaching a tube section to a pre-body a having the bottom wall from which the core extends such that the tube section forms the lateral wall around the core.
 10. The method according to claim 1, further comprising the step of manufacturing a plurality of hollow bodies and placing the hollow bodies on top of each other in the capsule.
 11. The method according to claim 10, wherein the bottom wall of each hollow body includes a female centering means and wherein the top wall of each hollow body includes a male centering means, wherein the female and male centering means are arranged such that a male centering means can be fully received in a female centering means.
 12. The method according to claim 11, wherein the bottom wall and/or the top wall of the plurality of hollow bodies are provided with a coating that prevents bonding of adjacent hollow bodies.
 13. The method according to claim 11, further comprising the step of placing a first cover piece in the capsule and then positioning the hollow bodies on top of each other in the capsule, wherein the cover element has a first surface which is directed towards the bottom of the capsule and a second surface which includes a protrusion which is arranged to be fully received in a female centering means.
 14. The method according to claim 1, wherein the metal machining apparatus is a lathe. 